This invention relates generally to an ionization vacuum gauge, and more particularly to a vacuum gauge employing a cold electron source.
Ionization vacuum pressure gauges have been built for more than fifty years. This type of gauge employs an electron source located radially outside of an ionization space defined by an anode. An ion collector electrode is disposed within the ionization space. The electrons travel from the source toward and through the anode, and are eventually collected by the anode. However, in their travel, the electrons impact the molecules and atoms of gas, constituting the atmosphere whose pressure is to be measured, and create ions. The ions are attracted by the collector electrode. The pressure of the gas within the atmosphere can be calculated by the formula P=(1/k) (Iion/Ielectron), where k is a constant with the units of 1/torr and is characteristic of a particular gauge geometry and electrical parameters.
One conventional prior art ionization vacuum gauge employs a hot filament for generating the ions. FIG. 1 is a schematic perspective illustration of such a prior art vacuum gauge. The ion gauge is disposed within an enclosure 11 whose pressure is to be measured. The gauge includes a heated filament 12 which emits the electrons, an anode grid 13 which accelerates the electrons and an ion collector 14 which collects ions formed by electron/molecule collisions. In the example, the anode grid is in the form of a wire spiral or screen which defines the ionization space and surrounds the collector. Ions are formed by collision of the electrons with the gas atoms or molecules within the enclosure whose pressure is being measured. Pressure is measured by collecting the ions, measuring the resulting electrical current and calculating the pressure as described above.
While this type of prior art vacuum gauge has been used successfully for many years, it has disadvantages and shortcomings. The hot filament requires several watts of electrical power which is dissipated as heat in the vacuum system. This heat changes the rate at which gas molecules are desorbed from the surfaces of the various structures in the vacuum system. It also changes the nature of the residual gases in the vacuum system by providing a hot source where gas molecules react by dissociation. When the hot ionization gauge is turned on in a high vacuum system, both the level and makeup of the vacuum pressure are disturbed as long as the gauge is operated. The radial location of the hot filament provides a dimensional limitation for the insertion of the gauge into the vacuum system. The hot source must not contact the port or tube into which the gauge is inserted and its proximity to the walls of the tube or port encourage excessive heating by radiation. The accuracy of the geometric relationship of the heated filament with the other gauge elements is difficult to maintain, because the filament can easily bend. This influences the accuracy of the measurements being made by the gauge. The high temperature of the filament causes evaporation which can give rise to undesirable coating of sensitive electrical components, such as insulators. In some cases, where the filament is coated with an active metal oxide, such as thorium oxide, certain materials, such as hydrocarbons, can give rise to xe2x80x98poisoningxe2x80x99 of the filament by dissociation and deposit of carbonaceous coatings. These residues change the electrical characteristics of the filament, making it unsuitable for its intended purpose. In other cases, where the filament is made of tungsten, exposure to high pressures of oxygen can cause destructive oxidation leading to burn-out.
U.S. Pat. No. 5,278,510 discloses a vacuum gauge which overcomes many of the drawbacks of the heated filament vacuum gauge. In this gauge, the heated filament electron source 12 is replaced by a cold micropoint electron source 16. FIG. 2 shows such a gauge where reference numerals have been applied to like parts. The cold source is radially mounted outside the ionizing space. This gives rise to the same geometric limitations as the hot filament electron sources.
It is an object of the present invention to provide an improved ionization vacuum pressure gauge.
The vacuum gauge includes an anode which defines an ionization space supported by a support. A cold electron source projects electrons axially directly through the support into the ionization space. An ion collector is supported within the ionization space.